The invention concerns film die heads for the production of single- or multi-layer film. Similar types of film die heads are already known. Film die heads for the production of single-layer tubular film consist of one central mandrel surrounded concentrically by a jacket. Located between the mandrel and the jacket is an annular slit through which the melt is transported to the ring-shaped die. On its outside circumferential surface, the mandrel has one or more spiral grooves, the spiral distributor. The depth of the grooves decreases from the start of the annular slit towards the ring-shaped die. The melt is conveyed by means of one or more melt feed units to the starting points of the grooves of the spiral distributor so that part of the melt is transported in the spiral grooves within the die head towards the ring-shaped die. As a result of the decreasing groove depth, an increasing amount of melt is conveyed over the flight land which separate the grooves in axial direction (see, e.g., U.S. Pat. Nos. 6,565,347 and 3,930,768 or their German equivalents, DE 199 24 540 C1 and DE 23 06 834).
Film die heads for multi-layer tubular film differ from those for single-layer tubular film in that the central mandrel is surrounded coaxially by several ring elements. The ring elements display spiral distributors on their outside circumferential surfaces. Annular slits form between the individual ring elements. The different annular slits are united upstream of the ring-shaped die. The distributor spirals of the individual ring elements are also connected to one or more melt feed units to ensure that they are supplied with melt (see, e.g., U.S. Pat. No. 3,966,377 or its German equivalent DE 23 20 687 and German patent DE 195 21 026).
Another design example of a film die head for multi-layer tubular film is described in (see, e.g., U.S. Pat. No. 6,702,563 or its European equivalent EP 1 055 504 B1). With this variant, the ring elements are conical in shape and are arranged on top of each other. In each case, two contra-oriented spiral channels whose depth decreases towards the opening are machined into the inside or outside circumferential surface of the conical ring elements.
Another variant of a film die head design is described in (see, e.g., U.S. patent application 2004/0166192 or its German equivalent DE 20307412 U1). Here, the spiral distributors are machined into both the inside delimiting wall and the outside delimiting wall of at least one melt channel. This serves to prevent film contamination caused by specks. The spirals of the inside and the outside delimiting walls of the melt channels can be arranged either in congruent or staggered form.
The function of the spiral distributor is to distribute the melt uniformly in the annular slit. The melt is split up by means of primary distributors into a number of separate streams which all flow into the channels of the spiral distributor. The melt stream that flows in the spirals is continuously divided into a tangential component which follows the path of the spiral and an axial component which flows over the flight land located between two spirals into the gap between two ring elements towards the ring-shaped die. As a result, melt which originates from a superimposition of the axial and tangential melt streams from all distributor channels flows at all points across the circumference of the ring-shaped die. These two components cause the melt streams of neighboring spiral channels to not only make contact along their edges but also to overlap each other over a large area of contact. Because of this, and because no joint lines arise with this system, a high thermal homogeneity can be achieved besides the desired mechanical homogeneity. By the end of the spiral distributor, the radial flow in the channels has turned into an exclusively axial flow towards the ring-shaped die.
To make sure an axial spiral flow sets in, a starting gap must be preset on a level with the starting zone of the spiral distributor. Here, part of the melt flows abruptly from the starting zone of the spiral channel into the annular slit. In the process, the melt flows along the production-dictated starting edge and is thus subjected to exceptional orientation and shearing. This fast-flowing axial melt stream displaces areas of melt of the channel above, which causes a sharply delineated boundary zone to form between the melt streams, so-called port lines. This has a negative effect on the quality of the film bubble. Because polymers have a memory function, an area of the film that was adversely affected by, for example, high stress or temperature can be recognized as a dull streak or wavy area. Thus, improvements in these type devices are needed and desired.